The 1/4-inch (6.25 mm) phone jack has become almost standardized as a signal connector for electrical musical instruments, such as electric guitars, electric keyboards and the like, and in many other applications. Despite the widespread use of such connectors, the performance and reliability of current 1/4-inch phone jack sockets is not satisfactory. Also, such phone jack sockets can be difficult to replace after they have failed or become noisy.
Current 1/4-inch phone jack sockets used in electrical musical instruments typically have a molded plastic body with an axial bore dimensioned to receive the jack plug. A number of elongate spring contact strips of different lengths are mounted in the body, each at a different point on the circumference of the bore. The contact strips lie along the axis and pass through axial slots in the rear face of the body. Each contact strip is anchored in the body part-way along its length by the axial slot. The end of the contact strip remote from the anchored end is bent in a complex profile to enable the contact strip to form a point contact with the jack plug and to ride over the surface of the jack plug as the latter is inserted into the jack socket. The contact strips are spring loaded towards the axis to enable them to form a positive contact with the jack plug. The contact strip must apply a substantial lateral force to the jack plug to reduce the electrical resistance of the contact between the contact strip and the jack plug despite the small area of contact between the contact strip and the jack plug.
The axial bore in the body of the jack socket is usually made generously large to reduce the force required to insert the jack plug against the friction resulting from the pressure that the contact strips exert on the jack plug. However, after the jack plug has been inserted, it is normally subject to both static and dynamic loads. The weight of the cable connected to the jack plug imposes a load on the jack plug in the direction perpendicular to the axial bore. This causes the jack plug to pitch relative to the jack socket about an axis perpendicular to the axial bore. With a sufficient load, and especially when the contact strips have lost some of their resilience, the jack plug can pitch sufficiently to break the contact between it and at least one of the contact strips. Even if the contact is not actually broken, the force between the contact strip and the jack plug can be reduced to such an extent that noise will be generated in response to the dynamic loads imposed by the performer's movements.
Each contact strip forms an almost point contact with a point on the jack plug. The small area of the point contact makes it vulnerable to environmental contamination. Such contamination can occur, for example, when the musical instrument in which the jack socket is installed is played in high temperature, high humidity conditions. Contamination of the point contact between the contact strip and the jack plug can result in a non-ohmic electrical connection between these elements. The electrical connection could be insulating, rectifying, or galvanic, for example. A non-ohmic electrical connection will degrade the quality of the signal generated by the musical instrument. The possibility of the electrical connection being non-ohmic is increased when the lateral force between the contact strip and the jack plug is small.
Jack sockets traditionally have two contact strips. This is sufficient to provide a single output channel. Recently, many musical instruments have been adapted to generate signals in more than one output channel so that multi-channel effects can be produced. Also, many musical instruments are now fitted with battery-powered pre-amplifiers so that they can generate an output signal having a high signal-to-noise ratio even when they employ high-impedance transducers, or even when they are connected to their respective amplifiers by long cables. To prolong battery life, it is desirable that the pre-amplifier operate only when the jack is plugged into the jack socket. This prevents the pre-amplifier from drawing current from the battery while the instrument is not in use.
These developments have increased the number of contacts that must be provided by the jack socket. To provide two output channels, three contacts are required. To switch the pre-amplifier on automatically when the jack is plugged into the jack socket requires a fourth contact. Jack sockets with three contact strips are common, but the reliability problems discussed above are exacerbated if a fourth contact strip is included because the width of the contact strips must be reduced to enable the fourth contact strip fit in the fixed circumference of the bore. Reducing the width of the contact strip reduces the contact pressure that the contact strip can exert on the jack plug, and reduces the resistance of the contact strip to lengthways buckling when the jack plug is inserted.
Jack sockets are not only less reliable than is desirable, but also can be difficult to replace when they fail. Jack sockets are conventionally secured in a mounting hole by an external nut engaging with the body surrounding the axial bore, or are formed with a flange surrounding the axial bore and are secured by a nut engaging with threads on the back of the body. Because of this, replacing a failed jack socket requires access to both the inside face and the outside face bounding the mounting hole in which the jack socket is mounted.
For example, the jack socket is normally installed in an acoustic guitar by replacing the strap peg on the end of the guitar with a combined strap peg and jack socket. The jack socket is mounted in an approximately 1/2" (12.5 mm) diameter mounting hole made at the former location of the strap peg and extending through the end block of the guitar. A typical jack socket adapted for this application has a threaded portion on the front of the jack socket body surrounding the axial bore onto which is screwed a flanged strap peg. The jack socket is secured in the mounting hole in the guitar body by a hexagonal nut that screws onto a second threaded portion on the back of the jack socket body.
Installing such a jack socket requires access to the interior of the body of the guitar. This is required so that the wires that are to be connected to the jack socket can be threaded through the nut and washer that will be engaged with the threads on the back of the jack socket. This is also required so that the nut and washer can be threaded onto the back of the jack socket, the jack socket inserted into the mounting hole, and the jack socket held to prevent it from rotating while the strap peg is tightened up. This procedure involves working with one hand inserted through the sound hole into the body of the guitar. It also requires that the strings be removed so that the hand can be inserted into the sound hole. After the jack socket has been installed and the strings replaced, the guitar must then be completely re-tuned and, sometimes, re-voiced.
Accordingly, a jack socket is required that has greater reliability than currently-available jack sockets. A positive contact to the jack plug should be provided irrespective of the direction of any static load applied to the jack socket, and the positive contact should be maintained regardless of what dynamic loads are applied, for example, as a result of the movements of the performer. The positive contact should be maintained after hundreds of thousands of cycles of inserting and removing the jack plug. Further, a jack socket that is required that remains highly reliable even when as many as four contacts are provided. Finally, a jack socket is required that can easily be installed in a mounting hole without the need for more access to the rear of the mounting hole than is provided by the mounting hole itself.